Internal combustion engines are used in various industrial applications for converting heat energy into mechanical energy. In motor vehicles, in particular heavy-goods vehicles, internal combustion engines are used to move the motor vehicle. The efficiency of internal combustion engines can be increased through the use of a system for utilizing waste heat of the internal combustion engine by means of the Rankine cycle. Such system converts waste heat of the internal combustion engine into mechanical energy. A known system includes a circuit having conduits for a working medium, for instance, water or an organic refrigerant such as R245fa, a pump for conveying the working medium, an evaporator heat exchanger for evaporating the liquid working medium, an expansion machine, a condenser for liquefying the evaporated working medium, and a collecting and compensating tank for the liquid working medium. Through the use of such systems in an internal combustion engine, the overall efficiency of the engine may be significantly increased.
In the evaporator heat exchanger, the working medium is evaporated using the waste heat of the engine, passed to the expansion machine, and expanded therein, performing a mechanical work delivered by the expansion machine as kinetic energy. In a typical evaporator heat exchanger, the working fluid is guided through a first conduit whereas the exhaust gas flow of the engine is guided through a second conduit. In this scenario, the heat from the exhaust gas may climb to a temperature in the range between 200° C. to 600° C., which is partly transferred to the working medium in the evaporator heat exchanger, allowing the working fluid to change from its liquid into a vaporous state of aggregation.
For use as a working medium for the Rankine cycle, numerous substances may be taken into consideration. Some of these substances, especially ethanol and organic fluids, possess threshold temperatures above which they decompose into highly toxic constituents. With such working media, the Rankine cycle cannot be operated continuously, rendering the use of waste heat of an internal combustion engine for increasing the efficiency of the engine merely possible. Some substances with an especially high threshold temperature may however be considered preferable from a thermodynamic point of view, for example, compared to water, because they allow greater efficiencies to be achieved and certain risks, such as the freezing of water, to be mitigated. Some such working media possess threshold temperatures ranging from 250° C. up to 400 or 500° C. When operating the Rankine cycle using exhaust gas as an energy source, passing exhaust gas of an external combustion engine through an evaporator heat exchanger, thus vaporizing the working medium in the evaporator heat exchanger, a counter-current flow is typically employed. This means that the flow of the exhaust gas is guided in a direction opposite to that of the working fluid passing through the evaporator heat exchanger. This approach is necessary to allow maximum heating of the working medium for obtaining an optimum efficiency of the Rankine cycle. Guiding the media in such counter-current flow may cause the working medium to be heated up to the temperature of the exhaust gas entering the evaporator heat exchanger. While in such configuration the exhaust gas may climb to an inlet temperature ranging between 350° C. to 700° C., temperature in a conventional evaporator heat exchanger located in an exhaust tailpipe commonly does not exceed a maximum of 400° C. Such excessive heating of the working medium near its inlet may jeopardize its thermal resistance.
Although it is possible, by controlling the respective mass flows of exhaust gas and working medium, to maintain a working temperature beneath the given threshold, there remains a risk that, due to inhomogeneity of the working media in the evaporator heat exchanger, the threshold may still be exceeded locally. Even in such transient operating state, there is a risk of overheating the working medium, causing it to decompose.
WO 2009/089 885 A1 shows an exhaust gas installation that comprises an exhaust gas evaporator mounted downstream of an internal combustion engine of a motor vehicle. The exhaust gas evaporator has a sandwich-type structure wherein exhaust gas planes and coolant planes are alternately directly adjacently arranged, providing a very compact while very efficient exhaust gas evaporator.
DE 10 2009 022 865 A1 shows a Rankine cycle, having an inlet or injecting opening through which a medium is introduced into the cycle during stoppage, so that the medium replaces water in a sub-area of the cycle. A collecting vessel is provided with increased storage volume, and another collecting vessel accommodates water. Volume of the collecting vessels corresponds to volume of heat exchangers to be emptied. An air supply line and a water vapor line are attached at the injecting opening. A heating device is provided for producing water vapor.
An exhaust heat recovery heat exchanger is known from DE 10 2007 056 113 A1. This exchanger has a working fluid flow path extending through a housing between a working fluid inlet and a working fluid outlet, where the path includes a portion adjacent to the working fluid inlet and another portion spaced apart from the working fluid inlet. The flow of the working fluid along the latter portion of the working fluid flow path is parallel to the flow of the exhaust along an exhaust flow path adjacent to the latter portion of the working fluid's flow path.